The Origin of Species
Chapter 14: Recapitulation and Conclusion
by Charles Darwin
Recapitulation of the difficulties on the theory of Natural Selection -
Recapitulation of the general and special circumstances in its favour - Causes of the
general belief in the immutability of species - How far the theory of natural selection
may be extended - Effects of its adoption on the study of Natural history - Concluding
remarks |
As this whole volume is one long argument, it may be convenient to the
reader to have the leading facts and inferences briefly recapitulated.
That many and grave objections may be advanced against the theory of descent with
modification through natural selection, I do not deny. I have endeavoured to give to them
their full force. Nothing at first can appear more difficult to believe than that the more
complex organs and instincts should have been perfected not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable slight variations,
each good for the individual possessor. Nevertheless, this difficulty, though appearing to
our imagination insuperably great, cannot be considered real if we admit the following
propositions, namely, -- that gradations in the perfection of any organ or instinct, which
we may consider, either do now exist or could have existed, each good of its kind, -- that
all organs and instincts are, in ever so slight a degree, variable, -- and, lastly, that
there is a struggle for existence leading to the preservation of each profitable deviation
of structure or instinct. The truth of these propositions cannot, I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by what gradations many
structures have been perfected, more especially amongst broken and failing groups of
organic beings; but we see so many strange gradations in nature, as is proclaimed by the
canon, `Natura non facit saltum,' that we ought to be extremely cautious in saying that
any organ or instinct, or any whole being, could not have arrived at its present state by
many graduated steps. There are, it must be admitted, cases of special difficulty on the
theory of natural selection; and one of the most curious of these is the existence of two
or three defined castes of workers or sterile females in the same community of ants but I
have attempted to show how this difficulty can be mastered. With respect to the almost
universal sterility of species when first crossed, which forms so remarkable a contrast
with the almost universal fertility of varieties when crossed, I must refer the reader to
the recapitulation of the facts given at the end of the eighth chapter, which seem to me
conclusively to show that this sterility is no more a special endowment than is the
incapacity of two trees to be grafted together, but that it is incidental on
constitutional differences in the reproductive systems of the intercrossed species. We see
the truth of this conclusion in the vast difference in the result, when the same two
species are crossed reciprocally; that is, when one species is first used as the father
and then as the mother.
The fertility of varieties when intercrossed and of their mongrel offspring cannot be
considered as universal; nor is their very general fertility surprising when we remember
that it is not likely that either their constitutions or their reproductive systems should
have been profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as domestication apparently
tends to eliminate sterility, we ought not to expect it also to produce sterility.
The sterility of hybrids is a very different case from that of first crosses, for their
reproductive organs are more or less functionally impotent; whereas in first crosses the
organs on both sides are in a perfect condition. As we continually see that organisms of
all kinds are rendered in some degree sterile from their constitutions having been
disturbed by slightly different and new conditions of life, we need not feel surprise at
hybrids being in some degree sterile, for their constitutions can hardly fail to have been
disturbed from being compounded of two distinct organisations. This parallelism is
supported by another parallel, but directly opposite, class of facts; namely, that the
vigour and fertility of all organic beings are increased by slight changes in their
conditions of life, and that the offspring of slightly modified forms or varieties acquire
from being crossed increased vigour and fertility. So that, on the one hand, considerable
changes in the conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of life and crosses
between less modified forms, increase fertility.
Turning to geographical distribution, the difficulties encountered on the theory of
descent with modification are grave enough. All the individuals of the same species, and
all the species of the same genus, or even higher group, must have descended from common
parents; and therefore, in however distant and isolated parts of the world they are now
found, they must in the course of successive generations have passed from some one part to
the others. We are often wholly unable even to conjecture how this could have been
effected. Yet, as we have reason to believe that some species have retained the same
specific form for very long periods, enormously long as measured by years, too much stress
ought not to be laid on the occasional wide diffusion of the same species; for during very
long periods of time there will always be a good chance for wide migration by many means.
A broken or interrupted range may often be accounted for by the extinction of the species
in the intermediate regions. It cannot be denied that we are as yet very ignorant of the
full extent of the various climatal and geographical changes which have affected the earth
during modern periods; and such changes will obviously have greatly facilitated migration.
As an example, I have attempted to show how potent has been the influence of the Glacial
period on the distribution both of the same and of representative species throughout the
world. We are as yet profoundly ignorant of the many occasional means of transport. With
respect to distinct species of the same genus inhabiting very distant and isolated
regions, as the process of modification has necessarily been slow, all the means of
migration will have been possible during a very long period; and consequently the
difficulty of the wide diffusion of species of the same genus is in some degree lessened.
As on the theory of natural selection an interminable number of intermediate forms must
have existed, linking together all the species in each group by gradations as fine as our
present varieties, it may be asked, Why do we not see these linking forms all around us?
Why are not all organic beings blended together in an inextricable chaos? With respect to
existing forms, we should remember that we have no right to expect (excepting in rare
cases) to discover directly connecting links between them, but only between each
and some extinct and supplanted form. Even on a wide area, which has during a long period
remained continuous, and of which the climate and other conditions of life change
insensibly in going from a district occupied by one species into another district occupied
by a closely allied species, we have no just right to expect often to find intermediate
varieties in the intermediate zone. For we have reason to believe that only a few species
are undergoing change at any one period; and all changes are slowly effected. I have also
shown that the intermediate varieties which will at first probably exist in the
intermediate zones, will be liable to be supplanted by the allied forms on either hand;
and the latter, from existing in greater numbers, will generally be modified and improved
at a quicker rate than the intermediate varieties, which exist in lesser numbers; so that
the intermediate varieties will, in the long run, be supplanted and exterminated.
On this doctrine of the extermination of an infinitude of connecting links, between the
living and extinct inhabitants of the world, and at each successive period between the
extinct and still older species, why is not every geological formation charged with such
links? Why does not every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? We meet with no such evidence, and this is
the most obvious and forcible of the many objections which may be urged against my theory.
Why, again, do whole groups of allied species appear, though certainly they often falsely
appear, to have come in suddenly on the several geological stages? Why do we not find
great piles of strata beneath the Silurian system, stored with the remains of the
progenitors of the Silurian groups of fossils? For certainly on my theory such strata must
somewhere have been deposited at these ancient and utterly unknown epochs in the world's
history.
I can answer these questions and grave objections only on the supposition that the
geological record is far more imperfect than most geologists believe. It cannot be
objected that there has not been time sufficient for any amount of organic change; for the
lapse of time has been so great as to be utterly inappreciable by the human intellect. The
number of specimens in all our museums is absolutely as nothing compared with the
countless generations of countless species which certainly have existed. We should not be
able to recognise a species as the parent of any one or more species if we were to examine
them ever so closely, unless we likewise possessed many of the intermediate links between
their past or parent and present states; and these many links we could hardly ever expect
to discover, owing to the imperfection of the geological record. Numerous existing
doubtful forms could be named which are probably varieties; but who will pretend that in
future ages so many fossil links will be discovered, that naturalists will be able to
decide, on the common view, whether or not these doubtful forms are varieties? As long as
most of the links between any two species are unknown, if any one link or intermediate
variety be discovered, it will simply be classed as another and distinct species. Only a
small portion of the world has been geologically explored. Only organic beings of certain
classes can be preserved in a fossil condition, at least in any great number. Widely
ranging species vary most, and varieties are often at first local, -- both causes
rendering the discovery of intermediate links less likely. Local varieties will not spread
into other and distant regions until they are considerably modified and improved; and when
they do spread, if discovered in a geological formation, they will appear as if suddenly
created there, and will be simply classed as new species. Most formations have been
intermittent in their accumulation; and their duration, I am inclined to believe, has been
shorter than the average duration of specific forms. Successive formations are separated
from each other by enormous blank intervals of time; for fossiliferous formations, thick
enough to resist future degradation, can be accumulated only where much sediment is
deposited on the subsiding bed of the sea. During the alternate periods of elevation and
of stationary level the record will be blank. During these latter periods there will
probably be more variability in the forms of life; during periods of subsidence, more
extinction.
With respect to the absence of fossiliferous formations beneath the lowest Silurian
strata, I can only recur to the hypothesis given in the ninth chapter. That the geological
record is imperfect all will admit; but that it is imperfect to the degree which I
require, few will be inclined to admit. If we look to long enough intervals of time,
geology plainly declares that all species have changed; and they have changed in the
manner which my theory requires, for they have changed slowly and in a graduated manner.
We clearly see this in the fossil remains from consecutive formations invariably being
much more closely related to each other, than are the fossils from formations distant from
each other in time.
Such is the sum of the several chief objections and difficulties which may justly be
urged against my theory; and I have now briefly recapitulated the answers and explanations
which can be given to them. I have felt these difficulties far too heavily during many
years to doubt their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor do we know how
ignorant we are. We do not know all the possible transitional gradations between the
simplest and the most perfect organs; it cannot be pretended that we know all the varied
means of Distribution during the long lapse of years, or that we know how imperfect the
Geological Record is. Grave as these several difficulties are, in my judgement they do not
overthrow the theory of descent with modification.
Now let us turn to the other side of the argument. Under domestication we see much
variability. This seems to be mainly due to the reproductive system being eminently
susceptible to changes in the conditions of life so that this system, when not rendered
impotent, fails to reproduce offspring exactly like the parent-form. Variability is
governed by many complex laws, -- by correlation of growth, by use and disuse, and by the
direct action of the physical conditions of life. There is much difficulty in ascertaining
how much modification our domestic productions have undergone; but we may safely infer
that the amount has been large, and that modifications can be inherited for long periods.
As long as the conditions of life remain the same, we have reason to believe that a
modification, which has already been inherited for many generations, may continue to be
inherited for an almost infinite number of generations. On the other hand we have evidence
that variability, when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently domesticated productions.
Man does not actually produce variability; he only unintentionally exposes organic
beings to new conditions of life, and then nature acts on the organisation, and causes
variability. But man can and does select the variations given to him by nature, and thus
accumulate them in any desired manner. He thus adapts animals and plants for his own
benefit or pleasure. He may do this methodically, or he may do it unconsciously by
preserving the individuals most useful to him at the time, without any thought of altering
the breed. It is certain that he can largely influence the character of a breed by
selecting, in each successive generation, individual differences so slight as to be quite
inappreciable by an uneducated eye. This process of selection has been the great agency in
the production of the most distinct and useful domestic breeds. That many of the breeds
produced by man have to a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal species.
There is no obvious reason why the principles which have acted so efficiently under
domestication should not have acted under nature. In the preservation of favoured
individuals and races, during the constantly-recurrent Struggle for Existence, we see the
most powerful and ever-acting means of selection. The struggle for existence inevitably
follows from the high geometrical ratio of increase which is common to all organic beings.
This high rate of increase is proved by calculation, by the effects of a succession of
peculiar seasons, and by the results of naturalisation, as explained in the third chapter.
More individuals are born than can possibly survive. A grain in the balance will determine
which individual shall live and which shall die, -- which variety or species shall
increase in number, and which shall decrease, or finally become extinct. As the
individuals of the same species come in all respects into the closest competition with
each other, the struggle will generally be most severe between them; it will be almost
equally severe between the varieties of the same species, and next in severity between the
species of the same genus. But the struggle will often be very severe between beings most
remote in the scale of nature. The slightest advantage in one being, at any age or during
any season, over those with which it comes into competition, or better adaptation in
however slight a degree to the surrounding physical conditions, will turn the balance.
With animals having separated sexes there will in most cases be a struggle between the
males for possession of the females. The most vigorous individuals, or those which have
most successfully struggled with their conditions of life, will generally leave most
progeny. But success will often depend on having special weapons or means of defence, or
on the charms of the males; and the slightest advantage will lead to victory.
As geology plainly proclaims that each land has undergone great physical changes, we
might have expected that organic beings would have varied under nature, in the same way as
they generally have varied under the changed conditions of domestication. And if there be
any variability under nature, it would be an unaccountable fact if natural selection had
not come into play. It has often been asserted, but the assertion is quite incapable of
proof, that the amount of variation under nature is a strictly limited quantity. Man,
though acting on external characters alone and often capriciously, can produce within a
short period a great result by adding up mere individual differences in his domestic
productions; and every one admits that there are at least individual differences in
species under nature. But, besides such differences, all naturalists have admitted the
existence of varieties, which they think sufficiently distinct to be worthy of record in
systematic works. No one can draw any clear distinction between individual differences and
slight varieties; or between more plainly marked varieties and subspecies, and species.
Let it be observed how naturalists differ in the rank which they assign to the many
representative forms in Europe and North America.
If then we have under nature variability and a powerful agent always ready to act and
select, why should we doubt that variations in any way useful to beings, under their
excessively complex relations of life, would be preserved, accumulated, and inherited?
Why, if man can by patience select variations most useful to himself, should nature fail
in selecting variations useful, under changing conditions of life, to her living products?
What limit can be put to this power, acting during long ages and rigidly scrutinising the
whole constitution, structure, and habits of each creature, -- favouring the good and
rejecting the bad? I can see no limit to this power, in slowly and beautifully adapting
each form to the most complex relations of life. The theory of natural selection, even if
we looked no further than this, seems to me to be in itself probable. I have already
recapitulated, as fairly as I could, the opposed difficulties and objections: now let us
turn to the special facts and arguments in favour of the theory.
On the view that species are only strongly marked and permanent varieties, and that
each species first existed as a variety, we can see why it is that no line of demarcation
can be drawn between species, commonly supposed to have been produced by special acts of
creation, and varieties which are acknowledged to have been produced by secondary laws. On
this same view we can understand how it is that in each region where many species of a
genus have been produced, and where they now flourish, these same species should present
many varieties; for where the manufactory of species has been active, we might expect, as
a general rule, to find it still in action; and this is the case if varieties be incipient
species. Moreover, the species of the large genera, which afford the greater number of
varieties or incipient species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the species of smaller
genera. The closely allied species also of the larger genera apparently have restricted
ranges, and they are clustered in little groups round other species -- in which respects
they resemble varieties. These are strange relations on the view of each species having
been independently created, but are intelligible if all species first existed as
varieties.
As each species tends by its geometrical ratio of reproduction to increase inordinately
in number; and as the modified descendants of each species will be enabled to increase by
so much the more as they become more diversified in habits and structure, so as to be
enabled to seize on many and widely different places in the economy of nature, there will
be a constant tendency in natural selection to preserve the most divergent offspring of
any one species. Hence during a long-continued course of modification, the slight
differences, characteristic of varieties of the same species, tend to be augmented into
the greater differences characteristic of species of the same genus. New and improved
varieties will inevitably supplant and exterminate the older, less improved and
intermediate varieties; and thus species are rendered to a large extent defined and
distinct objects. Dominant species belonging to the larger groups tend to give birth to
new and dominant forms; so that each large group tends to become still larger, and at the
same time more divergent in character. But as all groups cannot thus succeed in increasing
in size, for the world would not hold them, the more dominant groups beat the less
dominant. This tendency in the large groups to go on increasing in size and diverging in
character, together with the almost inevitable contingency of much extinction, explains
the arrangement of all the forms of life, in groups subordinate to groups, all within a
few great classes, which we now see everywhere around us, and which has prevailed
throughout all time. This grand fact of the grouping of all organic beings seems to me
utterly inexplicable on the theory of creation.
As natural selection acts solely by accumulating slight, successive, favourable
variations, it can produce no great or sudden modification; it can act only by very short
and slow steps. Hence the canon of `Natura non facit saltum,' which every fresh addition
to our knowledge tends to make more strictly correct, is on this theory simply
intelligible. We can plainly see why nature is prodigal in variety, though niggard in
innovation. But why this should be a law of nature if each species has been independently
created, no man can explain.
Many other facts are, as it seems to me, explicable on this theory. How strange it is
that a bird, under the form of woodpecker, should have been created to prey on insects on
the ground; that upland geese, which never or rarely swim, should have been created with
webbed feet; that a thrush should have been created to dive and feed on sub-aquatic
insects; and that a petrel should have been created with habits and structure fitting it
for the life of an auk or grebe! and so on in endless other cases. But on the view of each
species constantly trying to increase in number, with natural selection always ready to
adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in
nature, these facts cease to be strange, or perhaps might even have been anticipated.
As natural selection acts by competition, it adapts the inhabitants of each country
only in relation to the degree of perfection of their associates; so that we need feel no
surprise at the inhabitants of any one country, although on the ordinary view supposed to
have been specially created and adapted for that country, being beaten and supplanted by
the naturalised productions from another land. Nor ought we to marvel if all the
contrivances in nature be not, as far as we can judge, absolutely perfect; and if some of
them be abhorrent to our ideas of fitness. We need not marvel at the sting of the bee
causing the bee's own death; at drones being produced in such vast numbers for one single
act, and being then slaughtered by their sterile sisters; at the astonishing waste of
pollen by our fir-trees; at the instinctive hatred of the queen bee for her own fertile
daughters; at ichneumonidae feeding within the live bodies of caterpillars; and at other
such cases. The wonder indeed is, on the theory of natural selection, that more cases of
the want of absolute perfection have not been observed.
The complex and little known laws governing variation are the same, as far as we can
see, with the laws which have governed the production of so-called specific forms. In both
cases physical conditions seem to have produced but little direct effect; yet when
varieties enter any zone, they occasionally assume some of the characters of the species
proper to that zone. In both varieties and species, use and disuse seem to have produced
some effect; for it is difficult to resist this conclusion when we look, for instance, at
the logger-headed duck, which has wings incapable of flight, in nearly the same condition
as in the domestic duck; or when we look at the burrowing tucutucu, which is occasionally
blind, and then at certain moles, which are habitually blind and have their eyes covered
with skin; or when we look at the blind animals inhabiting the dark caves of America and
Europe. In both varieties and species correction of growth seems to have played a most
important part, so that when one part has been modified other parts are necessarily
modified. In both varieties and species reversions to long-lost characters occur. How
inexplicable on the theory of creation is the occasional appearance of stripes on the
shoulder and legs of the several species of the horse-genus and in their hybrids! How
simply is this fact explained if we believe that these species have descended from a
striped progenitor, in the same manner as the several domestic breeds of pigeon have
descended from the blue and barred rock-pigeon!
On the ordinary view of each species having been independently created, why should the
specific characters, or those by which the species of the same genus differ from each
other, be more variable than the generic characters in which they all agree? Why, for
instance, should the colour of a flower be more likely to vary in any one species of a
genus, if the other species, supposed to have been created independently, have differently
coloured flowers, than if all the species of the genus have the same coloured flowers? If
species are only well-marked varieties, of which the characters have become in a high
degree permanent, we can understand this fact; for they have already varied since they
branched off from a common progenitor in certain characters, by which they have come to be
specifically distinct from each other; and therefore these same characters would be more
likely still to be variable than the generic characters which have been inherited without
change for an enormous period. It is inexplicable on the theory of creation why a part
developed in a very unusual manner in any one species of a genus, and therefore, as we may
naturally infer, of great importance to the species, should be eminently liable to
variation; but, on my view, this part has undergone, since the several species branched
off from a common progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But a part may be
developed in the most unusual manner, like the wing of a bat, and yet not be more variable
than any other structure, if the part be common to many subordinate forms, that is, if it
has been inherited for a very long period; for in this case it will have been rendered
constant by long-continued natural selection.
Glancing at instincts, marvellous as some are, they offer no greater difficulty than
does corporeal structure on the theory of the natural selection of successive, slight, but
profitable modifications. We can thus understand why nature moves by graduated steps in
endowing different animals of the same class with their several instincts. I have
attempted to show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt sometimes comes into play in
modifying instincts; but it certainly is not indispensable, as we see, in the case of
neuter insects, which leave no progeny to inherit the effects of long-continued habit. On
the view of all the species of the same genus having descended from a common parent, and
having inherited much in common, we can understand how it is that allied species, when
placed under considerably different conditions of life, yet should follow nearly the same
instincts; why the thrush of South America, for instance, lines her nest with mud like our
British species. On the view of instincts having been slowly acquired through natural
selection we need not marvel at some instincts being apparently not perfect and liable to
mistakes, and at many instincts causing other animals to suffer.
If species be only well-marked and permanent varieties, we can at once see why their
crossed offspring should follow the same complex laws in their degrees and kinds of
resemblance to their parents, -- in being absorbed into each other by successive crosses,
and in other such points, -- as do the crossed offspring of acknowledged varieties. On the
other hand, these would be strange facts if species have been independently created, and
varieties have been produced by secondary laws.
If we admit that the geological record is imperfect in an extreme degree, then such
facts as the record gives, support the theory of descent with modification. New species
have come on the stage slowly and at successive intervals; and the amount of change, after
equal intervals of time, is widely different in different groups. The extinction of
species and of whole groups of species, which has played so conspicuous a part in the
history of the organic world, almost inevitably follows on the principle of natural
selection; for old forms will be supplanted by new and improved forms. Neither single
species nor groups of species reappear when the chain of ordinary generation has once been
broken. The gradual diffusion of dominant forms, with the slow modification of their
descendants, causes the forms of life, after long intervals of time, to appear as if they
had changed simultaneously throughout the world. The fact of the fossil remains of each
formation being in some degree intermediate in character between the fossils in the
formations above and below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct organic beings belong to the same system
with recent beings, falling either into the same or into intermediate groups, follows from
the living and the extinct being the offspring of common parents. As the groups which have
descended from an ancient progenitor have generally diverged in character, the progenitor
with its early descendants will often be intermediate in character in comparison with its
later descendants; and thus we can see why the more ancient a fossil is, the oftener it
stands in some degree intermediate between existing and allied groups. Recent forms are
generally looked at as being, in some vague sense, higher than ancient and extinct forms;
and they are in so far higher as the later and more improved forms have conquered the
older and less improved organic beings in the struggle for life. Lastly, the law of the
n='448'> long endurance of allied forms on the same continent, -- of marsupials in
Australia, of edentata in America, and other such cases, -- is intelligible, for within a
confined country, the recent and the extinct will naturally be allied by descent.
Looking to geographical distribution, if we admit that there has been during the long
course of ages much migration from one part of the world to another, owing to former
climatal and geographical changes and to the many occasional and unknown means of
dispersal, then we can understand, on the theory of descent with modification, most of the
great leading facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and in their
geological succession throughout time; for in both cases the beings have been connected by
the bond of ordinary generation, and the means of modification have been the same. We see
the full meaning of the wonderful fact, which must have struck every traveller, namely,
that on the same continent, under the most diverse conditions, under heat and cold, on
mountain and lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they will generally be descendants of the same progenitors
and early colonists. On this same principle of former migration, combined in most cases
with modification, we can understand, by the aid of the Glacial period, the identity of
some few plants, and the close alliance of many others, on the most distant mountains,
under the most different climates; and likewise the close alliance of some of the
inhabitants of the sea in the northern and southern temperate zones, though separated by
the whole intertropical ocean. Although two areas may present the same physical conditions
of life, we need feel no surprise at their inhabitants being widely different, if they
have been for a long period completely separated from each other; for as the relation of
organism to organism is the most important of all relations, and as the two areas will
have received colonists from some third source or from each other, at various periods and
in different proportions, the course of modification in the two areas will inevitably be
different.
On this view of migration, with subsequent modification, we can see why oceanic islands
should be inhabited by few species, but of these, that many should be peculiar. We can
clearly see why those animals which cannot cross wide spaces of ocean, as frogs and
terrestrial mammals, should not inhabit oceanic islands; and why, on the other hand, new
and peculiar species of bats, which can traverse the ocean, should so often be found on
islands far distant from any continent. Such facts as the presence of peculiar species of
bats, and the absence of all other mammals, on oceanic islands, are utterly inexplicable
on the theory of independent acts of creation.
The existence of closely allied or representative species in any two areas, implies, on
the theory of descent with modification, that the same parents formerly inhabited both
areas; and we almost invariably find that wherever many closely allied species inhabit two
areas, some identical species common to both still exist. Wherever many closely allied yet
distinct species occur, many doubtful forms and varieties of the same species likewise
occur. It is a rule of high generality that the inhabitants of each area are related to
the inhabitants of the nearest source whence immigrants might have been derived. We see
this in nearly all the plants and animals of the Galapagos archipelago, of Juan Fernandez,
and of the other American islands being related in the most striking manner to the plants
and animals of the neighbouring American mainland; and those of the Cape de Verde
archipelago and other African islands to the African mainland. It must be admitted that
these facts receive no explanation on the theory of creation.
The fact, as we have seen, that all past and present organic beings constitute one
grand natural system, with group subordinate to group, and with extinct groups often
falling in between recent groups, is intelligible on the theory of natural selection with
its contingencies of extinction and divergence of character. On these same principles we
see how it is, that the mutual affinities of the species and genera within each class are
so complex and circuitous. We see why certain characters are far more serviceable than
others for classification; -- why adaptive characters, though of paramount importance to
the being, are of hardly any importance in classification; why characters derived from
rudimentary parts, though of no service to the being, are often of high classificatory
value; and why embryological characters are the most valuable of all. The real affinities
of all organic beings are due to inheritance or community of descent. The natural system
is a genealogical arrangement, in which we have to discover the lines of descent by the
most permanent characters, however slight their vital importance may be.
The framework of bones being the same in the hand of a man, wing of a bat, fin of the
porpoise, and leg of the horse, -- the same number of vertebrae forming the neck of the
giraffe and of the elephant, -- and innumerable other such facts, at once explain
themselves on the theory of descent with slow and slight successive modifications. The
similarity of pattern in the wing and leg of a bat, though used for such different
purposes, -- in the jaws and legs of a crab, -- in the petals, stamens, and pistils of a
flower, is likewise intelligible on the view of the gradual modification of parts or
organs, which were alike in the early progenitor of each class. On the principle of
successive variations not always supervening at an early age, and being inherited at a
corresponding not early period of life, we can clearly see why the embryos of mammals,
birds, reptiles, and fishes should be so closely alike, and should be so unlike the adult
forms. We may cease marvelling at the embryo of an air-breathing mammal or bird having
branchial slits and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiae.
Disuse, aided sometimes by natural selection, will often tend to reduce an organ, when
it has become useless by changed habits or under changed conditions of life; and we can
clearly understand on this view the meaning of rudimentary organs. But disuse and
selection will generally act on each creature, when it has come to maturity and has to
play its full part in the struggle for existence, and will thus have little power of
acting on an organ during early life; hence the organ will not be much reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited teeth, which never
cut through the gums of the upper jaw, from an early progenitor having well-developed
teeth; and we may believe, that the teeth in the mature animal were reduced, during
successive generations, by disuse or by the tongue and palate having been fitted by
natural selection to browse without their aid; whereas in the calf, the teeth have been
left untouched by selection or disuse, and on the principle of inheritance at
corresponding ages have been inherited from a remote period to the present day. On the
view of each organic being and each separate organ having been specially created, how
utterly inexplicable it is that parts, like the teeth in the embryonic calf or like the
shrivelled wings under the soldered wing-covers of some beetles, should thus so frequently
bear the plain stamp of inutility! Nature may be said to have taken pains to reveal, by
rudimentary organs and by homologous structures, her scheme of modification, which it
seems that we wilfully will not understand.
I have now recapitulated the chief facts and considerations which have thoroughly
convinced me that species have changed, and are still slowly changing by the preservation
and accumulation of successive slight favourable variations. Why, it may be asked, have
all the most eminent living naturalists and geologists rejected this view of the
mutability of species? It cannot be asserted that organic beings in a state of nature are
subject to no variation; it cannot be proved that the amount of variation in the course of
long ages is a limited quantity; no clear distinction has been, or can be, drawn between
species and well-marked varieties. It cannot be maintained that species when intercrossed
are invariably sterile, and varieties invariably fertile; or that sterility is a special
endowment and sign of creation. The belief that species were immutable productions was
almost unavoidable as long as the history of the world was thought to be of short
duration; and now that we have acquired some idea of the lapse of time, we are too apt to
assume, without proof, that the geological record is so perfect that it would have
afforded us plain evidence of the mutation of species, if they had undergone mutation.
But the chief cause of our natural unwillingness to admit that one species has given
birth to other and distinct species, is that we are always slow in admitting any great
change of which we do not see the intermediate steps. The difficulty is the same as that
felt by so many geologists, when Lyell first insisted that long lines of inland cliffs had
been formed, and great valleys excavated, by the slow action of the coast-waves. The mind
cannot possibly grasp the full meaning of the term of a hundred million years; it cannot
add up and perceive the full effects of many slight variations, accumulated during an
almost infinite number of generations.
Although I am fully convinced of the truth of the views given in this volume under the
form of an abstract, I by no means expect to convince experienced naturalists whose minds
are stocked with a multitude of facts all viewed, during a long course of years, from a
point of view directly opposite to mine. It is so easy to hide our ignorance under such
expressions as the `plan of creation,' `unity of design,' &c., and to think that we
give an explanation when we only restate a fact. Any one whose disposition leads him to
attach more weight to unexplained difficulties than to the explanation of a certain number
of facts will certainly reject my theory. A few naturalists, endowed with much flexibility
of mind, and who have already begun to doubt on the immutability of species, may be
influenced by this volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with impartiality.
Whoever is led to believe that species are mutable will do good service by conscientiously
expressing his conviction; for only thus can the load of prejudice by which this subject
is overwhelmed be removed.
Several eminent naturalists have of late published their belief that a multitude of
reputed species in each genus are not real species; but that other species are real, that
is, have been independently created. This seems to me a strange conclusion to arrive at.
They admit that a multitude of forms, which till lately they themselves thought were
special creations, and which are still thus looked at by the majority of naturalists, and
which consequently have every external characteristic feature of true species, -- they
admit that these have been produced by variation, but they refuse to extend the same view
to other and very slightly different forms. Nevertheless they do not pretend that they can
define, or even conjecture, which are the created forms of life, and which are those
produced by secondary laws. They admit variation as a vera causa in one case, they
arbitrarily reject it in another, without assigning any distinction in the two cases. The
day will come when this will be given as a curious illustration of the blindness of
preconceived opinion. These authors seem no more startled at a miraculous act of creation
than at an ordinary birth. But do they really believe that at innumerable periods in the
earth's history certain elemental atoms have been commanded suddenly to flash into living
tissues? Do they believe that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants created as eggs or
seed, or as full grown? and in the case of mammals, were they created bearing the false
marks of nourishment from the mother's womb? Although naturalists very properly demand a
full explanation of every difficulty from those who believe in the mutability of species,
on their own side they ignore the whole subject of the first appearance of species in what
they consider reverent silence.
It may be asked how far I extend the doctrine of the modification of species. The
question is difficult to answer, because the more distinct the forms are which we may
consider, by so much the arguments fall away in force. But some arguments of the greatest
weight extend very far. All the members of whole classes can be connected together by
chains of affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very wide intervals
between existing orders. Organs in a rudimentary condition plainly show that an early
progenitor had the organ in a fully developed state; and this in some instances
necessarily implies an enormous amount of modification in the descendants. Throughout
whole classes various structures are formed on the same pattern, and at an embryonic age
the species closely resemble each other. Therefore I cannot doubt that the theory of
descent with modification embraces all the members of the same class. I believe that
animals have descended from at most only four or five progenitors, and plants from an
equal or lesser number.
Analogy would lead me one step further, namely, to the belief that all animals and
plants have descended from some one prototype. But analogy may be a deceitful guide.
Nevertheless all living things have much in common, in their chemical composition, their
germinal vesicles, their cellular structure, and their laws of growth and reproduction. We
see this even in so trifling a circumstance as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly produces monstrous
growths on the wild rose or oak-tree. Therefore I should infer from analogy that probably
all the organic beings which have ever lived on this earth have descended from some one
primordial form, into which life was first breathed.
When the views entertained in this volume on the origin of species, or when analogous
views are generally admitted, we can dimly foresee that there will be a considerable
revolution in natural history. Systematists will be able to pursue their labours as at
present; but they will not be incessantly haunted by the shadowy doubt whether this or
that form be in essence a species. This I feel sure, and I speak after experience, will be
no slight relief. The endless disputes whether or not some fifty species of British
brambles are true species will cease. Systematists will have only to decide (not that this
will be easy) whether any form be sufficiently constant and distinct from other forms, to
be capable of definition; and if definable, whether the differences be sufficiently
important to deserve a specific name. This latter point will become a far more essential
consideration than it is at present; for differences, however slight, between any two
forms, if not blended by intermediate gradations, are looked at by most naturalists as
sufficient to raise both forms to the rank of species. Hereafter we shall be compelled to
acknowledge that the only distinction between species and well-marked varieties is, that
the latter are known, or believed, to be connected at the present day by intermediate
gradations, whereas species were formerly thus connected. Hence, without quite rejecting
the consideration of the present existence of intermediate gradations between any two
forms, we shall be led to weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally acknowledged to be
merely varieties may hereafter be thought worthy of specific names, as with the primrose
and cowslip; and in this case scientific and common language will come into accordance. In
short, we shall have to treat species in the same manner as those naturalists treat
genera, who admit that genera are merely artificial combinations made for convenience.
This may not be a cheering prospect; but we shall at least be freed from the vain search
for the undiscovered and undiscoverable essence of the term species.
The other and more general departments of natural history will rise greatly in
interest. The terms used by naturalists of affinity, relationship, community of type,
paternity, morphology, adaptive characters, rudimentary and aborted organs, &c., will
cease to be metaphorical, and will have a plain signification. When we no longer look at
an organic being as a savage looks at a ship, as at something wholly beyond his
comprehension; when we regard every production of nature as one which has had a history;
when we contemplate every complex structure and instinct as the summing up of many
contrivances, each useful to the possessor, nearly in the same way as when we look at any
great mechanical invention as the summing up of the labour, the experience, the reason,
and even the blunders of numerous workmen; when we thus view each organic being, how far
more interesting, I speak from experience, will the study of natural history become!
A grand and almost untrodden field of inquiry will be opened, on the causes and laws of
variation, on correlation of growth, on the effects of use and disuse, on the direct
action of external conditions, and so forth. The study of domestic productions will rise
immensely in value. A new variety raised by man will be a far more important and
interesting subject for study than one more species added to the infinitude of already
recorded species. Our classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of creation. The rules
for classifying will no doubt become simpler when we have a definite object in view. We
possess no pedigrees or armorial bearings; and we have to discover and trace the many
diverging lines of descent in our natural genealogies, by characters of any kind which
have long been inherited. Rudimentary organs will speak infallibly with respect to the
nature of long-lost structures. Species and groups of species, which are called aberrant,
and which may fancifully be called living fossils, will aid us in forming a picture of the
ancient forms of life. Embryology will reveal to us the structure, in some degree
obscured, of the prototypes of each great class.
When we can feel assured that all the individuals of the same species, and all the
closely allied species of most genera, have within a not very remote period descended from
one parent, and have migrated from some one birthplace; and when we better know the many
means of migration, then, by the light which geology now throws, and will continue to
throw, on former changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the inhabitants of the
whole world. Even at present, by comparing the differences of the inhabitants of the sea
on the opposite sides of a continent, and the nature of the various inhabitants of that
continent in relation to their apparent means of immigration, some light can be thrown on
ancient geography.
The noble science of Geology loses glory from the extreme imperfection of the record.
The crust of the earth with its embedded remains must not be looked at as a well-filled
museum, but as a poor collection made at hazard and at rare intervals. The accumulation of
each great fossiliferous formation will be recognised as having depended on an unusual
concurrence of circumstances, and the blank intervals between the successive stages as
having been of vast duration. But we shall be able to gauge with some security the
duration of these intervals by a comparison of the preceding and succeeding organic forms.
We must be cautious in attempting to correlate as strictly contemporaneous two formations,
which include few identical species, by the general succession of their forms of life. As
species are produced and exterminated by slowly acting and still existing causes, and not
by miraculous acts of creation and by catastrophes; and as the most important of all
causes of organic change is one which is almost independent of altered and perhaps
suddenly altered physical conditions, namely, the mutual relation of organism to organism,
-- the improvement of one being entailing the improvement or the extermination of others;
it follows, that the amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time. A number of species,
however, keeping in a body might remain for a long period unchanged, whilst within this
same period, several of these species, by migrating into new countries and coming into
competition with foreign associates, might become modified; so that we must not overrate
the accuracy of organic change as a measure of time. During early periods of the earth's
history, when the forms of life were probably fewer and simpler, the rate of change was
probably slower; and at the first dawn of life, when very few forms of the simplest
structure existed, the rate of change may have been slow in an extreme degree. The whole
history of the world, as at present known, although of a length quite incomprehensible by
us, will hereafter be recognised as a mere fragment of time, compared with the ages which
have elapsed since the first creature, the progenitor of innumerable extinct and living
descendants, was created.
In the distant future I see open fields for far more important researches. Psychology
will be based on a new foundation, that of the necessary acquirement of each mental power
and capacity by gradation. Light will be thrown on the origin of man and his history.
Authors of the highest eminence seem to be fully satisfied with the view that each
species has been independently created. To my mind it accords better with what we know of
the laws impressed on matter by the Creator, that the production and extinction of the
past and present inhabitants of the world should have been due to secondary causes, like
those determining the birth and death of the individual. When I view all beings not as
special creations, but as the lineal descendants of some few beings which lived long
before the first bed of the Silurian system was deposited, they seem to me to become
ennobled. Judging from the past, we may safely infer that not one living species will
transmit its unaltered likeness to a distant futurity. And of the species now living very
few will transmit progeny of any kind to a far distant futurity; for the manner in which
all organic beings are grouped, shows that the greater number of species of each genus,
and all the species of many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to foretel that it will be
the common and widely-spread species, belonging to the larger and dominant groups, which
will ultimately prevail and procreate new and dominant species. As all the living forms of
life are the lineal descendants of those which lived long before the Silurian epoch, we
may feel certain that the ordinary succession by generation has never once been broken,
and that no cataclysm has desolated the whole world. Hence we may look with some
confidence to a secure future of equally inappreciable length. And as natural selection
works solely by and for the good of each being, all corporeal and mental endowments will
tend to progress towards perfection.
It is interesting to contemplate an entangled bank, clothed with many
plants of many kinds, with birds singing on the bushes, with various insects flitting
about, and with worms crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each other, and dependent on each other
in so complex a manner, have all been produced by laws acting around us. These laws, taken
in the largest sense, being Growth with Reproduction; inheritance which is almost implied
by reproduction; Variability from the indirect and direct action of the external
conditions of life, and from use and disuse; a Ratio of Increase so high as to lead to a
Struggle for Life, and as a consequence to Natural Selection, entailing Divergence of
Character and the Extinction of less-improved forms. Thus, from the war of nature, from
famine and death, the most exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur in this view of
life, with its several powers, having been originally breathed into a few forms or into
one; and that, whilst this planet has gone cycling on according to the fixed law of
gravity, from so simple a beginning endless forms most beautiful and most wonderful have
been, and are being, evolved.